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Title: The evolution of stellar mass and morphology in the last 11 billion years
Author: Mortlock, Alice
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2013
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This thesis explores the stellar mass growth of galaxies and the bimodality of galaxy morphologies and properties throughout the last 11 billion years of cosmic time. The data is from three deep, near infrared surveys: the GOODS NICMOS Survery (GNS), the Ultra Deep Survey (UDS) and the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). Firstly, we use a sample of 8298 galaxies observed as part of the GNS using the Hubble Space Telescope (BST) H16o-band. We construct the galaxy stellar mass function both as a function of redshift and stellar mass in the redshift range 1 < z < 3.5. We find that a significant fraction of all massive (111* > 10111110) galaxies are in place up to the highest red shifts we probe, with a decreasing fraction of lower mass galaxies present at all redshifts. This is consistent with the downsizing scenario where the most massive galaxies have built up their stellar mass before low mass galaxies. By examining the stellar mass function divided into blue/red systems, as well as for star forming and nonstar forming galaxies, we find a similar mass down sizing present for these populations. We fit the mass functions with a Schechter function and find that the faint end slope parameter values range from ex = -1.36 to -1. 73, which is significantly steeper than what is found in previous studies in the literature. We compute the stellar mass density by integrating the Schechter functions that we have fit and find our results to be higher than those in the literature. As a result, the steeper mass function better matches the stellar mass added due to star formation, thereby alleviating some of the discord between these two measures of the evolution of galaxy mass. We construct the total galaxy stellar mass function, in the red shift range 0.3 < z < 3.0, using the full UDS and CANDELS UDS data sets. The combination of these two Abstract 3 surveys gives us excellent number statistics from the full UDS (more than 100,000 galaxies over the 0.88 deg2 field). Furthermore, we have the depth to probe the low mass end of the galaxy stellar mass function from the CANDELS UDS (50' depth of H=26.3 in a 1" -aperture). We find similar results to the GNS total mass functions, including similar values of a despite deeper data. Furthermore, we divide the galaxy stellar mass function by environment and colour. We show that the growth of quiescent galaxies could be driven by mass and environment dependent quenching processes out to a redshift of z rv 2. Finally, we examine 1188 massive galaxies with JVI* ~ 1010 M0 between redshifts Z = 1- 3 within the CANDELS UDS. Using this sample we determine how galaxy structure and morphology evolve with time, and investigate the nature of galaxy structure at high redshift. We visually classify our sample into disks, ellipticals and peculiar systems, conecting for redshift effects on these classifications through simulations. We find significant evolution in the fractions of galaxies at a given visual classification as a function of redshift. The peculiar popUlation is dominant at Z > 2 with a substantial spheroid population, and a negligible disk population. We compute the transition redshift, Ztrans , where the combined fraction of spheroidal and disk galaxies is equal to that of the peculiar population, as Ztrans = 1.86 ± 0.62 for galaxies in our stellar mass range. We find that this transition changes as a function of stellar mass, with Hubble-type galaxies becoming dominant at higher redshifts for higher mass galaxies ( Ztrans = 2.22 ± 0.82), than for the lower mass galaxies (Ztrans = 1.73 ± 0.57). Higher mass galaxies become morphologically settled before their lower mass counterparts, a form of morphological downsizing. We furthermore compare our visual classifications with Sersic index, concentration, asymmetry and clumpiness (CAS) parameters, star formation rate and rest frame U - B colour. We find links between the colour of a galaxy, its star formation rate and how extended or peculiar it appears. We discuss the negligible Z > 2 disk fraction based on visual morph~logies and speculate that this is due to disks in formation appearing peculiar through processes such as violent disk instabilities or mergers: To properly define high redshift morphology and structure we suggest that a new and more exact classification scheme is needed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available